1. Field of the Invention
The present invention provides a voltage-controlled amplifier, and more particularly, a voltage-controlled amplifier with low noise, low distortion, high integration degree, and low cost.
2. Description of the Prior Art
A voltage-controlled amplifier, or VCA, can be seen as a three-terminal unit, including an input end, an output end, and a control signal reception end, utilized for changing a gain according to a control signal. The VCA is usually used in a multimedia electric device, such as a DVD player, a VCD player, a portable video player, a digital TV, etc. A user can adjust volume, contrast, brightness, or a channel of the multimedia electric device through the VCA.
The VCA has become more and more important owing to a variety of signal types. The prior art VCAs (voltage-controlled amplifiers) are manufactured with a bipolar process or a BiCMOS (bipolar complementary metal oxide semiconductor) process. A VCA, manufactured with the bipolar process, can operate in a high speed and drive a high current, but cannot be integrated with a digital circuit. In comparison, a VCA, manufactured with the BiCMOS process, combines advantages of a BJT and a CMOS, including high speed, high current driving, low power, high input impedance, high noise margin, etc. However, cost of the BiCMOS manufacturing process is too high. Therefore, a VCA with low noise, low distortion, high integration degree, and low cost is needed.